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<title>Local Register Variables (Using the GNU Compiler Collection (GCC))</title>

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<div class="subsubsection-level-extent" id="Local-Register-Variables">
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Previous: <a href="Global-Register-Variables.html" accesskey="p" rel="prev">Defining Global Register Variables</a>, Up: <a href="Explicit-Register-Variables.html" accesskey="u" rel="up">Variables in Specified Registers</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html" title="Index" rel="index">Index</a>]</p>
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<h4 class="subsubsection" id="Specifying-Registers-for-Local-Variables"><span>6.47.5.2 Specifying Registers for Local Variables<a class="copiable-link" href="#Specifying-Registers-for-Local-Variables"> &para;</a></span></h4>
<a class="anchor" id="Local-Reg-Vars"></a><a class="index-entry-id" id="index-local-variables_002c-specifying-registers"></a>
<a class="index-entry-id" id="index-specifying-registers-for-local-variables"></a>
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<p>You can define a local register variable and associate it with a specified 
register like this:
</p>
<div class="example smallexample">
<pre class="example-preformatted">register int *foo asm (&quot;r12&quot;);
</pre></div>

<p>Here <code class="code">r12</code> is the name of the register that should be used.  Note
that this is the same syntax used for defining global register variables, 
but for a local variable the declaration appears within a function.  The 
<code class="code">register</code> keyword is required, and cannot be combined with 
<code class="code">static</code>.  The register name must be a valid register name for the
target platform.
</p>
<p>Do not use type qualifiers such as <code class="code">const</code> and <code class="code">volatile</code>, as
the outcome may be contrary to expectations. In particular, when the
<code class="code">const</code> qualifier is used, the compiler may substitute the
variable with its initializer in <code class="code">asm</code> statements, which may cause
the corresponding operand to appear in a different register.
</p>
<p>As with global register variables, it is recommended that you choose 
a register that is normally saved and restored by function calls on your 
machine, so that calls to library routines will not clobber it.
</p>
<p>The only supported use for this feature is to specify registers
for input and output operands when calling Extended <code class="code">asm</code> 
(see <a class="pxref" href="Extended-Asm.html">Extended Asm - Assembler Instructions with C Expression Operands</a>).  This may be necessary if the constraints for a 
particular machine don&rsquo;t provide sufficient control to select the desired 
register.  To force an operand into a register, create a local variable 
and specify the register name after the variable&rsquo;s declaration.  Then use 
the local variable for the <code class="code">asm</code> operand and specify any constraint 
letter that matches the register:
</p>
<div class="example smallexample">
<pre class="example-preformatted">register int *p1 asm (&quot;r0&quot;) = ...;
register int *p2 asm (&quot;r1&quot;) = ...;
register int *result asm (&quot;r0&quot;);
asm (&quot;sysint&quot; : &quot;=r&quot; (result) : &quot;0&quot; (p1), &quot;r&quot; (p2));
</pre></div>

<p><em class="emph">Warning:</em> In the above example, be aware that a register (for example 
<code class="code">r0</code>) can be call-clobbered by subsequent code, including function 
calls and library calls for arithmetic operators on other variables (for 
example the initialization of <code class="code">p2</code>).  In this case, use temporary 
variables for expressions between the register assignments:
</p>
<div class="example smallexample">
<pre class="example-preformatted">int t1 = ...;
register int *p1 asm (&quot;r0&quot;) = ...;
register int *p2 asm (&quot;r1&quot;) = t1;
register int *result asm (&quot;r0&quot;);
asm (&quot;sysint&quot; : &quot;=r&quot; (result) : &quot;0&quot; (p1), &quot;r&quot; (p2));
</pre></div>

<p>Defining a register variable does not reserve the register.  Other than
when invoking the Extended <code class="code">asm</code>, the contents of the specified 
register are not guaranteed.  For this reason, the following uses 
are explicitly <em class="emph">not</em> supported.  If they appear to work, it is only 
happenstance, and may stop working as intended due to (seemingly) 
unrelated changes in surrounding code, or even minor changes in the 
optimization of a future version of gcc:
</p>
<ul class="itemize mark-bullet">
<li>Passing parameters to or from Basic <code class="code">asm</code>
</li><li>Passing parameters to or from Extended <code class="code">asm</code> without using input 
or output operands.
</li><li>Passing parameters to or from routines written in assembler (or
other languages) using non-standard calling conventions.
</li></ul>

<p>Some developers use Local Register Variables in an attempt to improve 
gcc&rsquo;s allocation of registers, especially in large functions.  In this 
case the register name is essentially a hint to the register allocator.
While in some instances this can generate better code, improvements are
subject to the whims of the allocator/optimizers.  Since there are no
guarantees that your improvements won&rsquo;t be lost, this usage of Local
Register Variables is discouraged.
</p>
<p>On the MIPS platform, there is related use for local register variables 
with slightly different characteristics (see <a data-manual="gccint" href="https://gcc.gnu.org/onlinedocs/gccint/MIPS-Coprocessors.html#MIPS-Coprocessors">Defining coprocessor specifics for MIPS targets</a> in <cite class="cite">GNU Compiler Collection (GCC) Internals</cite>).
</p>
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